https://www.martinfowler.com/bliki/AnemicDomainModel.html
"This layer is the heart of business software"
- POJOs
- combine data and process (behavior)
- domain logic and knowledge
https://www.martinfowler.com/eaaCatalog/serviceLayer.html
- Defines jobs
- orchestratees domain objects to do the job
- to be kept thin
- doen't contain business rules or knowledge
- only coordinates tasks
- "delegates work to collaborations of domain objects"